A simplified diagrammatic representation of a disk drive, generally designated as 10, is illustrated in FIG. 1. The disk drive 10 includes a data storage disk 12 that is rotated by a spindle motor 14. The spindle motor 14 is mounted to a base plate 16. An actuator arm assembly 18 is also mounted to the base plate 16.
The actuator arm assembly 18 includes a read/write head 20 mounted to a flexure arm 22 which is attached to an actuator arm 24 that can rotate about a pivot bearing assembly 26. The actuator arm assembly 18 also includes a voice coil motor (VCM) 28 which moves the head 20 relative to tracks defined on the disk 12. The spindle motor 14, VCM 28, and head 20 are coupled to a number of electronic circuits 30 mounted to a printed circuit board 32. Although a single disk 12 is illustrated in FIG. 1, the disk drive 10 may instead include a plurality of disks with a head adjacent to each disk storage surface to read/write therefrom.
FIG. 2 is an exemplary top view of the disk 12. Data is stored on the disk 12 within a number of concentric tracks 40 (or cylinders). Each track is divided into a plurality of radially extending sectors 42 of the disk 12. Each sector 42 is further divided into a servo sector 44 and a data sector 46. Information in the servo sectors 44 is used to, among other things, accurately position the head 20 so that host data can be properly written onto and read from the data sectors 46.
FIG. 3 illustrates exemplary servo information 73 within servo information that may be stored in each of the servo sectors 44. The servo information 73 can include a DC erase field 731, a preamble field 732, a servo address mark (SAM) field 733, a track number field indicated by its least significant bits (LSBs) 734, a spoke number field 735, an entire track number field 736 which is recorded in at least one of the servo sectors 44, and a servo burst field 737 of circumferentially staggered radially offset servo bursts (e.g., A, B, C, D servo bursts).
A read write channel circuit in the electronic circuits 30 may use the DC erase field 731 as an indication of the onset of one of the servo sectors 44. The channel circuit may use the preamble field 732 to establish a gain and disk-locked timing relationship for sampling the analog signal that is generated when reading from the disk 12. More particularly, the channel circuit may generate a servo clock signal based on the preamble field 732. The channel circuit may count cycles of the servo clock between adjacent SAMs 733 and may adjust the frequency of the servo clock signal in an attempt to provide a substantially constant number of cycles between adjacent SAMs 733. The channel circuit may divide the servo clock signal by a counter to generate a lower frequency clock signal. The lower frequency clock signal may be used to estimate the starting and stopping locations of the data sectors 46. Because of uncertainty in the estimated locations of the fields of the servo sectors 44 and/or the data sectors 46, the length of the DC erase field 731, the preamble field 732, other fields and/or gaps may need to be increased. As can be appreciated, such lengthening of the fields/gaps can decrease the storage capacity and/or the read/write throughput of the disk drive 10.